1. Field of the Invention
Generally, the field of the present invention is laser diode apparatuses.
2. Background
The advent of the semiconductor diode laser has enabled significant advancement in a plethora of technological areas, including the industrial, consumer, and medical arenas.
Even before semiconductor diode lasers matured economically, the idea of producing an ultra-high power, ultra-high density laser beam was disclosed in U.S. Pat. No. 4,828,357 to Arata et al. Separate laser beams are reflected by respective mirrors to form a densely packed bundle of parallel beams or are directed to converge in a particular location. However, directing semiconductor diode lasers into a concentrated beam is significantly more complicated due to anamorphic beam characteristics of the constituent diode lasers, and the size scale and tolerances of the component parts required to fabricate the diode module. Moreover, since many diode modules are fiber-coupled, fiber characteristics such as acceptance angle and aperture size become additional constraints in the design of the module and component parts that provide light to the fiber.
The beam quality of laser diodes is almost diffraction limited with an M2 value close to 1 in the plane perpendicular to the active region, while the beam quality of a broad area device in the plane parallel to the active region is relatively poor with an M2 value typically around 20-30. Furthermore, dead-space between emitters in a laser diode bar array can reduce the M2 to values of around 1000. This asymmetry in beam quality between the fast and slow axes is a primary challenge that needs to be overcome for the use of laser diodes in fiber coupled systems. This is typically achieved by rearranging the array of emitters in the slow axis and stacking them in the fast axis, resulting in a system with matched beam quality in the fast and slow axes.
For example, in U.S. Pat. No. 5,319,528 to Raven, a high power light source is disclosed that includes an array of laser diode modules optically coupled to a beam shaping and combining unit that has anamorphic prism pairs and that optically manipulates and directs beams to flat reflectors. The reflected beams are arranged in a stack that is directed to a focusing lens and subsequently coupled into an optical fiber. U.S. Pat. Nos. 7,733,932 and 8,000,360 to Faybishenko disclose an apparatus that includes a thermally dissipative body with stepped surfaces where upon each surface is mounted a laser diode structure emitting a laser beam parallel to the stepped surface, a slow axis collimation lens, and a beam reflecting turning mirror. Again the reflected beams are arranged in a stack that is directed to a focusing lens and subsequently coupled into an optical fiber.
In bar-based systems, the asymmetric beam quality in the fast and slow axes requires the use of expensive micro-optical beam shaping systems. These systems, which rotate the fast and slow axes of the individual emitters in the laser bar, are typically implemented with the use of step mirror arrays, arrays of micro-optical cylinder lens telescopes rotated by 45 degrees, or by lateral beam displacement techniques, such as those described in U.S. Pat. Nos. 5,986,794 and 6,462,883. While these systems are effective at rotating the optical axes, the optical to optical efficiency is diminished by multiple optical interfaces, imperfect beam rotation or stacking in the fast axis, and low brightness laser sources. The brightness of bar-based systems is further limited due to emitter cross heating and bar “smile.” Cross heating increases the effective thermal resistance, forcing the individual emitters within the diode laser bar to run at lower power densities to maintain reasonable reliabilities. Bar smile introduces fast axis pointing error and optical defocusing, further diminishing the beam quality of the system.
Thus, although several laser diode module designs have emerged over the past few decades, a need remains for a laser diode apparatus that reduces the number of elements used in the optical system or reduces the overall complexity, while maintaining the ability to individually address pointing and collimation of the individual laser beams, so that a module may be provided that is simpler to manufacture and that can exhibit improved etendue preservation, while providing high-power low-divergence output.